I know it isn't necessarily a valid comparison, but as a long time FFXI player, I don't feel Mog Bonanza trivialized the acquisition of these items. Many of the desirable items were Rare/Ex (Unique/Untradeable in FFXIV-speak). It wasn't like they were giving out Wivre mounts or anything. Sure, there were some tradeable items (Herald's Gaiters comes to mind), but keep in mind that just like FFXIV, gil wasn't really the limiting factor for players in FFXI. There were many ways to earn gil, but if you weren't in a large linkshell that farmed certain open-world contested enemies, you had 0% chance of getting these items. However, with many ways to earn gil, you could just buy them.

With that said, I don't know how SE could implement Mog Bonanza in FFXIV. Most items from ShB and earlier and relatively easy to obtain since you can just unsync the content. Maybe a voucher for any relic you want from a particular expansion? I suppose that could work. It is essentially glamour only and doesn't affect the marketboard at all. Something like that would cheapen the achievement though (for those that count that). How about a voucher to automatically win any housing in the lottery? That would be a great prize for some.